Color filters for imaging devices used heretofore have generally made use of absorption properties of colorants. However, such color filters using colorants must be thick to achieve sufficient absorption, and colors of the colorants fade with the passage of time.
On the other hand, pixels of imaging devices, such as charge coupling devices (CCDs) and complementary metal-oxide semiconductor (CMOS) image sensors, are becoming smaller with the increase in the number of pixels. In such a case, since the amount of light captured by detector units of image sensors decreases, the thickness of microlenses, color filters, wirings, etc., is desirably small to capture a sufficient amount of light. Moreover, since image sensors also capture obliquely incident light, color-filter characteristics that do not significantly vary with respect to the obliquely incident light are desired.
Under such circumstances, color filters that use a plasmon resonance phenomenon are being studied. Plasmon is collective oscillation of free electrons confined to metal surfaces and excited by external electrical fields, such as light. Since electrons are charged, oscillation of electrons results in polarization caused by a density distribution of free electrons. The coupling of such polarization to electromagnetic fields is called “plasmon resonance”. When light is incident on metal particles or metal structures, a resonance phenomenon is observed in which scattering or absorption increases in a particular wavelength band. This phenomenon is the localized surface plasmon resonance (simply referred to as “plasmon resonance” hereinafter) and the wavelength at which the absorption spectrum shows the maximum peak is referred to as a “plasmon resonance wavelength”.
As light is transmitted through metal particles or metal structures, the transmittance decreases in a wavelength band in which plasmon resonance occurs and selective transmission of light occurs, depending on the wavelength. Thus, this can be used to form color filters. Since a plasmon resonance phenomenon also occurs on thin metal structures, it possibly contributes to the thickness reduction of image sensors.
U.S. Pat. No. 5,973,316 discloses an array formed by periodically arranging apertures smaller than the wavelength of the incident light in a metal thin film. The period of the apertures and the size of the apertures are matched with excited plasmon to increase the transmittance in a particular wavelength band.
Nature Photonics (2008) 2, 161-164 teaches a structure including a periodic structure constituted by concentric grooves formed in a silver film and having an aperture at the center of the concentric circles and describes that this structure exhibits color-filter characteristics according to which the intensities of red, green, and blue transmitted light increase.
According to the aperture array disclosed in U.S. Pat. No. 5,973,316, the percentage of the metal that serves as a light-blocking portion with respect to the apertures is large and thus, the transmittance is only about 5% to 6% at most. The aperture array, thus, does not have a sensitivity sufficient for color-filter application.
According to the color filter incorporating the periodic structure in the silver film disclosed in Nature Photonics (2008) 2, 161-164, light is transmitted through the aperture at the center of the concentric grooves and thus a sufficient transmittance cannot be expected under current technologies. Moreover, in such a case, the color-filter characteristic requires a large region sufficient for ensuring periodicity of the metal structure. Thus, since the metal structure must be placed within a pixel and the size of one pixel is increasingly becoming smaller, a sufficient periodicity of the metal structure may not be ensured and the color-filter characteristic may deteriorate. Moreover, this literature does not focus on improving the overall characteristics of optical elements including a plurality of color filters having different characteristics.